Food Industry Wastewater Treatment
Our company – and its legal predecessor – has extensive experience in the food industry: we have built complete, chemical-dosing-based physico-chemical wastewater treatment technology for numerous dairy plants, slaughterhouses and meat processing plants. These solutions share a number of identical technological steps, regardless of the type of plant involved. A general treatment scheme may consist of the following elements:
Screening and mechanical pre-treatment
Wastewater from manufacturing processes – food industry wastewater in particular – can contain large quantities of solid pollutants. It is advisable to remove these from the system as early as possible, before they cause technological problems (e.g. blockages). Solid pollutants are usually removed mechanically, with screens and bar racks. Where possible, the extracted material can be recovered as a by-product; otherwise it must be handled and disposed of as waste.
Collection and homogenisation
Wastewater from manufacturing processes originates from various technological steps, so its quantity and quality can vary within wide limits. These fluctuations are evened out in the collection/homogenisation tank, which ensures a more consistent load for the subsequent treatment steps.
pH adjustment
pH adjustment is one of the generally applied steps of treatment technologies, and may be required both for chemical treatment and for biological degradation. It is typically carried out in two series-connected, stirred reactors: the first performs coarse adjustment, the second fine-tuning. Acidic and alkaline chemicals are stored in special (double-walled, bunded) chemical storage and dosing tanks designed for this purpose, and the dosing pumps are controlled by pH probes.
Coagulation
Coagulation takes place practically simultaneously with pH adjustment, in both space and time. Typical pollutants in food industry wastewater – fats, oils and proteins – are present in the water in colloidal, emulsified form; this colloidal stability is broken down by the iron- or aluminium-based coagulant. The aim is to remove the pollutants in solid form and bind them into large-surface-area metal hydroxide flocs (sludge flocs). In addition to the coagulation chemicals, we also plan to dose hydrated lime to neutralise the pH of the wastewater: this forms a calcium carbonate precipitate with the bicarbonate content, which becomes incorporated into the developing flocs, improving the later dewaterability of the sludge and thus the dry matter content achieved after dewatering.
Chemical background for professionals
The basic chemicals used in the process are acid-dissolved iron or aluminium salts, which dissolve in the wastewater and then precipitate as metal hydroxide at the neutral to slightly alkaline pH of the wastewater:
Fe(II)SO4 → Fe2+ + SO42-
Al(OH)xCl3-x → Al(OH)x- + (3-x) Cl-
Fe2+ + Ca2+ + 2 OH- → Fe(OH)2↓ + Ca2+ (pH>8,8)
Al3+ + 1,5 Ca2+ + 3 OH- → Al(OH)3↓ + 1,5 Ca2+ (pH>6,0)
The resulting metal hydroxide precipitates link together to form large flocs, which bind the colloidal organic pollutants in the water. Compared to sodium or potassium hydroxide, the hydrated lime used for pH neutralisation offers several practical advantages: it reacts with the bicarbonate present in the wastewater to form calcium carbonate,
Ca(OH)2 + 2 HCO3- → CaCO3↓ + 2 H2O
which becomes incorporated into the developing flocs, improving the water-releasing capacity of the sludge – resulting in a higher dry matter content at the later dewatering stage. For a similar purpose, we also plan to dose bentonite, which is made up of porous-structured particles. A further advantage of hydrated lime dosing is that, compared to sodium or potassium hydroxide, it increases the dissolved salt content of the water relatively little, since those reagents would form water-soluble salts with the bicarbonate.
Flocculation
The metal hydroxide and pollutant flocs (sludge) formed during coagulation are relatively small in size and behave in a stable manner in the wastewater due to their surface charge. To be separated from the water phase, their size must be increased – this is achieved by dosing the flocculant (also known as a polyelectrolyte). The flocculant is based on a polymer chain (e.g. polyacrylamide), whose length, charge and charge density are tailored to the required function during manufacture; it can be in powder or liquid form. The optimal type is always selected through laboratory testing, and is then dosed as a dilute (0.1–0.4%) aqueous solution, prepared in a polymer dissolving unit.
Phase separation and sludge dewatering
Separation of the water and sludge phases can be carried out using a sedimentation or flotation unit: in sedimentation, gravity forces the sludge flocs to the bottom of the unit, while in flotation, microbubbles released from air dissolved under pressure float them to the surface. The separated sludge is still very dilute (1–3%) at this stage, so it is dewatered in order to reduce disposal costs and transport costs. Commonly used solutions in practice include: plastic chamber filter presses, centrifuges, belt filter presses, and screw sludge presses. When selecting the equipment, we consider a number of factors (batch/continuous operation, energy demand, capacity, achievable dry matter content) – in our view, no single solution can be declared "the best" in advance: the optimum is always determined by the specific parameters of the given plant.
Wastewater discharge
A significant proportion of food industry wastewater can be treated using the chemical process described above, particularly when the treated water is discharged into the public sewer – in this case, the removal efficiency of dissolved organic matter (COD, chemical oxygen demand) is generally sufficient to ensure compliance with statutory discharge limits. If the receiving body is a natural watercourse, and the efficiency of the chemical (physico-chemical) treatment alone is not sufficient, the technology is supplemented with an additional, typically biological stage, which removes biologically degradable, non-toxic substances dissolved in the water phase.
If you would like to discuss the design, modernisation or capacity expansion of the wastewater treatment technology at your food industry plant, please contact us through one of our available channels.